Two resistors are in series if the current leaving the first resistor is undivided by branching before entering the second resistor. That is, you should be able to draw a line on the circuit diagram from one resistor to the other without hitting a point at which the current branches off.

Two resistors are in parallel if the current that enters each resistor flows from the same node or source without passing through any other elements and if the current that leaves each resistor flows to the same node or sink without passing through any other elements.